Noise suppression apparatus for telephones



Feb. 20, 1940. B. FISHER 2,191,375

NOISE SUPPRESSION APPARATUS FOR TELEPHONES Filed June 2, 1939 PatentedFeb. 20, 1940 UNITED STATES NOISE SUPPRESSION APPARATUS FOR TELEPHONESBergie L. Fisher, Martinsville, Va. Application June 2, 1939, Serial No.277,106

13 Claims.

This invention relates to a noise suppression system for telephonelines, generally, and in particular to open wire ground return telephonelines in rural communities afiected by power induction and staticdisturbances. The invention is also applicable to metallic trunk linesbetween exchanges where the length of the trunk line is comparativelyshort.

An object of this invention is to provide a relatively inexpensive noisesuppression system for telephone lines of the open wire ground returntype or the short trunk line type.

Another object of this invention is to provide a noise suppressionsystem for telephone lines adapted to utilize the commercial alternatingcurrent as a source of power.

A further object of this invention is to provide.

a noise suppression system for telephone lines adapted to impress thevoice currents upon the telephone line at high transmission levels toobtain a high voice to noise ratio without the use of circuits that willproduce troublesome sustained oscillations.

Still another object of this invention is to provide a noise suppressionsystem for telephone lines, said noise suppression system employing ahigh gain amplifier tube for amplifying to a high transmission level thevoice currents impressed upon the telephone line, said amplifier tubehaving the power circuits thereof energized from a commercial lightingcircuit or similar power source which is also connected to the telephoneline for bell ringing purposes.

A further object of this invention is to provide a noise suppressionsystem for telephone lines employing a high gain amplifier tube foramplifying the voiced currents impressed upon the telephone line to ahigh transmission level to obtain a high voiced to noise ratio, saidamplifier tube being adapted to have the power circuits thereofenergized from a commercial power or lighting circuit and being adaptedto have its anode or output circuit connected to said power sourcethrough a filtering impedance or choke coil which is also adapted tofunction as animpedance coupling for coupling said anode or outputcircuit to a telephone or transmission line.

Another object of this invention is to provide a noise suppressor oramplifier adapted for use on a telephone line and employing a vacuumtube amplifier which is adapted to be connected to the telephonecirciuit through resistance networks, said resistance network beingarranged to prevent sustained oscillations in the circuits ofarrangement for the suppression of noise in telephone lines particularlythe open wire ground return type of telephone-lines and also intelephone circuits employing comparatively short trunk lines betweenexchanges. The apparatus of my inventionis'comparatively simple,reliable in operation and inexpensive which makes it feasible for use inrural communities where telephone lines are afiected by power inductionand static disturbances. The telephone set employed in accordance withthis invention is provided with one or more amplifier tubes having themicrophone or transmitter connected to the input and the telephone lineconnected to the output for amplifying the signal from the microphone toa relatively high level .prior to impressing the signal on the telephoneline. Where one tube is employed in the amplifier I prefer to use thepentode type although of course any other type of amplifier tube may beused as will appear from the following specification. The amplifier tubeis suitably coupled to the telephone line and a rectifying tube devicefor rectifying the alternating current derived from the conventionallighting circuit is used to energizethe plate circuit of the amplifier.

The signal is transmitted over the telephone line to the receiver whereit is reproduced, It is intended that all telephones on the same line beequipped with this type or substantially this same type of noisesuppressor since a telephone set so equipped will not operate with a setnot so equipped. However, it would not be necessary to provide everyline in the telephone exchange with telephone sets of this high gaintype since one telephone line employing such sets may be coupled to aline employing conventional telephone sets through a noise suppressorcircuit which also forms a feature of this invention. This noisesuppressor circuit also includes a vacuum tube of the pentode typealthough, of course, other types of tubes well known in the art may beused. Pure resistances, that is, resistance devices consistingsubstantially only of ohmic resistance, are connected into an electricalnetwork both in the input circuit and output circuit of the noisesuppressor tube to permit two directions of transmission through thenoise suppressor. These and other features of this invention will bemore fully set forth in the following specification and the drawing inwhich briefly, Fig. 1 is a schematic circuit diagram of the noisesuppressor telephone set and Fig. 2 is a schematic circuit diagram ofthe noise suppressor circuit for connecting various telephone lines.

Referring to the drawing in detail reference numeral designates apentode or similar type of high gain amplifier tube having the controlgrid 3 thereof connected to the secondary 4 of the microphonetransformer 5. The telephone transmitter or microphone 2 is connected tothe primary 6 of the transformer and is electrically energized from thesecondary winding 1 of the transformer 6 through the bridge rectifyingcircuit 9 and the filter circuit including the choke coil I 0 and filtercapacity II. The filament I2 of the amplifier tube I is connected to thesecondary I3 of the transformer 8 and the center tap of this secondaryI3 is connected to the resistance and capacity circuit I4 to obtain thedesired grid bias for the grid electrode 3 with respect to the filamentor cathode I2. The circuit I4 including the bias resistor and the bypasscondenser is grounded-to the metal chassis or frame upon which theelements of this apparatus are supported and in this way the circuitbetween the lower terminal of the secondary 4 which is also grounded tothe chassis and the filament circuit is completed. The amplifier tube Iis also provided with a suppressor grid which is connected to thefilament and an additional grid I5 which is connected to the anode I1through the resistor I6 so that a relatively high positive potential isap plied to the grid I5. Positive potential derived from the secondaryI8 of the transformer 8 and the rectifier I9 is applied :to the anode I1of the amplifier I through the impedance and choke coil 20. The centertap of the secondary I8 is grounded to the chassis of the apparatus andthe two terminals of the secondary are connected to the two anodes 2|and 22 of the tube I9. The filament 23 of therectifier I9 is connectedto the secondary 24 of the transformer 8.

The filament of the rectifier I9 is connected to the anode I I of thetube I through the impedance of the choke coil 20 so that the positivepotential from the rectified alternating current supplyis applied to theanode of the amplifier. The negative terminal of the rectifierapparatus, namely the center tapof the secondary winding I8 is connectedor grounded to the metal frame or chassis as is also the resistor andcondenser cir-, cuit I4 so that the negative potential from therectifier is applied to the filament I2 of the amplifier tube throughthe circuit I4 and the secondary winding I3. The return circuit for thecontrol grid 3 to the filament I2 is also completed through thetransformer secondary winding I3 inasmuch as the secondary 4 of thetransformer 5 is also grounded to the chassis as indicated so that it isin effect connected to the rounded terminal of the resistance circuit I4and in this way the grid 3 is given a direct current biasing potentialdepending upon the potential difference across the resistor of circuitI4. The potential difference across this resistor will of course bedetermined by the current flowing through the tube I from the filamentI2. A filter condenser 25 is connected to the filament of the rectifierI9 and the choke coil 26 as shown in the drawing. The choke coil 20 isalso employed as an impedance for coupling the anode circuit of theamplifier I to the telephone line 36 so that this choke must be designedto prevent the passage of telephonic signals derived from the microphone2 and amplified by the amplifier I, from being shunted to the groundthrough the filter condenser 25. These highly amplified telephonesignals must be impressed upon the line 36 through a coupling condenser26 and in this case the telephone circuit employs one metallic conductor36 and the earth or ground as the return circuit. Thus the metallicframe or chassis of the telephone set is also connected to the earth orgrounded in order that this telephone line circuit may be properlycompleted.

It will be observed thatthe amplifier tube I is of the pentode typehaving three grid electrodes, namely the control grid 3, the screen gridI5 which is connected to the. anode I1 through a suitable resistance I6and a suppressor grid which is connected to the filament- It is obviousthat amplifier tubes other than the pentode type may be employed and ifnecessary more than one amplifier tube may be used.

The telephone receiver 3| is connected to the line 36 through the switch29 and the tap 32 of the resistor 30. The other terminal of the receiver3| is connected to the ground. The resistor 30 is employed for thepurpose of introducing a transmission loss between the receiver 3| andthe line substantially equal to the transmission gain of the pentodeamplifier. Therefore the tendency toward the production of sustainedelectrical oscillations caused by the acousticalmechanical couplingbetween the receiver 3| and the transmitter 2 is reduced to the samevalue as in a telephone set in which no amplifier is used. The advantageof this type of telephone set is that the voice currefits are deliveredto the line 36 at a high transmission level, traverse the line at a highlevel and are reduced to the normal telephone transmission level by theresistance pad 30 in the receiver circuit. thereby the ratio of noiselevel to voice transmission level is decreased by the transmission gainof the pentode amplifier.

/ Both of the switches 28 and 29 are arranged the 110 volt alternatingcurrent circuit to the transformer primary 33 is closed through theswitch 28 when the receiver 3| is removed from its supporting hook andthe telephone set is ready for operation inasmuch as the variouscircuits described above are energized from the various sec- 4 ondariesof the transformer 8.

The alternating current bell 21 as well as other similar bells connectedto the line 36 may be energized from the 110 volt alternating currentcircuit by closing the push-button 35 which is connected to the bell 21and line 36. A current limiting resistance 34 is connected between the110 volt line and the push-button switch 35. The lower terminal of thebell 21 is grounded and in this way it may be energized by closing theswitch 35 without closing the receiver operated switch 28. It is ofcourse obvious that if the telephone line 36 employs two metallic linesthat the bell However, it will not be necessary to so equip every linein the telephone exchange for by means of the noise suppressor describedbelow it is possible to connect a line equipped with noise suppressortelephone sets to a line using conventional telephone equipment. It isalso possible to use the noise suppressor described below on two wiremetallic trunk circuits of moderate length.

In Fig. 2 is shown a diagram of a noise suppressor circuit that may beemployed to connect a telephone line employing telephone sets such asshown in Fig. 1, to a telephone line employing conventional telephonesets. In this case the telephone line 36 is connected to the terminal 49of the resistance network a. employing 'resistors 45, 46, 41 and 48. Thesecondary 44 of the transformer 42 is also connected to the terminal 49and also to the terminal 5| of this network.

The terminal 52 is connected to the earth or ground in cases where theground is used as the return circuit or if a metallic return circuit isused then this terminal is connected to such a metallic circuit. Thisterminal 52 is connected by means of the wire 53 to the terminal 6| ofanother resistance network 54a. including the resistances 54, 55, 56 and51. The terminals 58 and 60 of the latter resistance network 5411 areconnected to the primary 64 of the transformer 62. The telephonelines 66and 61 employing conventional telephone equipment are connected to theterminals 58 and 59 of the latter resistance network. The two resistancenetworks are also connected together through the wire 69, the variableresistance 68 and wire 10.

In the circuit of Fig. 2 transformer 1| supplies the required voltagefor the vacuum tubes in the noise suppressor. The full wave electronicrectifier 16, retardation coil BI and condensers 82 and 83 supply thenecessary high voltage for the anode 4| of the pentodetube 31. Thewinding 13 also supplies the necessary cathode current for pentode tube31. Resistance 14 and condenser 14a are used to obtain the proper po-'tential relation between the cathode and control grid of the pentodetube.

The control grid electrode 39 of the pentode 31 is connected to thesecondary 63 of the transformer 62 and to the condenser 65 which isshunted across this secondary. The lower terminal of the transformersecondary 63 is grounded to the metal frame or chassis of the apparatusforming the grid to filament return circuit by way of'the resistance 14and secondary winding 13. The anode 4| of the pentode amplifier tube 31is connected to the primary 43 of the output transformer 42. I

Thus it is seen that the telephone signals coming from the lines 66 and61 are impressed upon the input circuit of the amplifier tube 31 andamplified whereupon they are transmitted to the line 36 at a relativelyhigh level. On the other hand signals coming from the line 36 thefamiliar bridge arrangement is used to separate the two directions oftransmission. The use of this arrangement instead of the usual specialtype transformers and complicated balancing impedance networks used inprevious systems of this natureis made possible by the fact that onground return circuits of the usual length employed the input impedanceis determined to a very large extent by the terminating impedance, whichin this case will be largely resistive depending upon the value ofresistance 36 in Fig. 1.

Likewise on two wire or metallic telephone circuits of open 'wireconstruction of moderate length the terminating impedance controls to alarge extent the input impedance of the line. Therefore, when two ofthese noise suppressors are connected, one at each end of a metallictrunk line between twoexchanges the bridge arrangement of each noisesuppressor is suificiently well balanced to prevent sustainedoscillation from being generated.

, A similar bridge arrangement is provided by a pure resistance network540. at the input of the noise suppressor tube 31 as was describedabove.

The operation of the two types of suppressors in combination is asfollows: C'onsider first transmission from a switchboard to a telephonestation. Voice currents from. a telephone line enter terminal 66-61 ofthe noise suppressor in Fig. 2 and are impressed upon the control gridof pentode tube 31 by input transformer 62. The voice currents areamplified by pentode tube 31 and impressed upon the resistance network45a by output transformer 42. Resistance network 45a prevents thesecurrents from entering circuit 53-16 but allows them to enter the line36 at the terminal. After traversing the line they enter resistance 30of Fig. 1 and after being attenuated by this resistance to a normaltelephone transmission level are introduced into receiver 3|.

Transmission in the opposite direction, that is from telephone stationtoswitchboard is accomplished as follows: The voice currents generatedin the carbon button transmitter set of Fig. 1 are amplified to a highlevel and impressed upon the line as described above. These voicecurrents traverse the line 36 and enter the terminal of Fig. 2. Uponentering resistance network 450: a part of the voice currents enter thecircuit 53-10 and a part are impressed on the secondary of outputtransformer 1 where they serve no useful purpose. The part of the voicecurrents entering the circuit 5310 flow through variable resistance 68,which serves the purpose of reducing the currents to the desiredtransmission level, and then into resistance network 54a where they areprevented from entering input transformer 62 because of the balancedcondition of the network, but are impressed across the lines 6661 at thenoise suppressor.

The amplified voice currents in traversing resistance networks 54a and45a. and variable resistance 68 have been attenuated to a normaltelephone level.

While I have described this invention in detail with respect to theforegoing embodiments thereof it is of course apparent that variousmodification may be made therein without departing from the spirit andscope of this invention and therefore I do not desire to limit thisinvention to the details set forth except insofar as they are defined bythe claims.

What I claim is as follows: v

1. Noise suppression apparatus for telephone lines, comprising: amicrophone, an amplifier having input and output circuits, connectionsbetween said microphone and said input circuit, a telephone lineconnected to said output circuit for receiving highly amplified signalsfrom said amplifier output circuit, a telephone receiver, and aresistance pad connected between said telephone line and said telephonereceiver, said resistance pad being adjusted to attenuate the signalsimpressed upon said telephone receiver to a lower level sufficient forreproduction of said signals by said telephone receiver so that theundesired noise is reduced so as to have substantially negligibleinterference effect on the desired signal, a second amplifier at aremote point from said first amplifier, said second amplifier havinginput and output circuits, a second telephone line connected to saidsecond amplifier input circuit for feeding signals to be amplified tosaid second amplifier, a resistance network connected between saidsecond amplifier output circuit and said first mentioned telephone linefor impressing amplified signals upon said first mentioned telephoneline, and an attenuating device connected between said resistancenetworks for passing telephonic signals therethrough from said firsttelephone line to said second telephone line at an attenuated level.

2. Noise suppression apparatus for telephone lines, comprising: anamplifier tube having cathode, grid and plate electrodes, a resistancenetwork connected between a telephone line and the plate circuit of saidamplifier tube, a second resistance network connected between the gridcircuit of said amplifier tube and a second tele-- phone line, means forconnecting said second resistance network to said grid circuit forimpressing telephone signals to be amplified upon said gridcircuitfmeans for connecting said first resistance network to said platecircuit for impressing highly amplified telephone signals from saidplate circuit upon said first telephone line, and an attenuating deviceconnected between said resistancenetworks for passing telephonic signalstherethrough from said first telephone line to said second telephoneline at an attenuated level.

3. Noise suppression apparatus for telephone lines, comprising: anamplifier tube having cathode, grid and plate electrodes, asubstantially balanced resistance network connected between a telephoneline and the plate circuit of said amplifier tube, a secondsubstantially balanced resistance network connected between the gridcircuit of said amplifier tube and a secondtelephone line, saidsubstantially balanced resistance networks being connected between theirrespective circuits so that telephone signals are normally transmittedthrough said amplifier from input to output and amplified to arelatively high transmission level to obtain a high signal to noiseratio, and an attenuating device connected between said resistancenetworks for passing telephonic signals therethrough from said firsttelephone line to said second telephone line and attenuate said signalsto a lower transmission level at the same time keeping the high signalto noise ratio obtaining 'on said first telephone line.

4. Noise suppression apparatus for telephone lines, comprising: anamplifier tube having an input circuit and an output circuit, a pair ofresistance bridge networks, one of said resistance bridge networks beingconnected to said amplifier input circuit for impressing signals from atelephone line upon said input circuit to be amplified and the other ofsaid resistance bridge networks being connected to said amplifier outputcircuit for impressing amplified signals from said output circuit upon atelephone line, said resistance bridge networks each having a pair ofpoints having substantially zero potential therebetween and means forconnecting the corresponding zero potential points of said bridgenetworks together whereby signal transmission in the direction oppositeto that normally accomplished through said amplifier tube from the inputto the output thereof may be accomplished around said amplifier tube.

5. Noise suppression apparatus for telephone lines, comprising: anamplifier tube having an input circuit and an output circuit, a pair ofresistance bridge networks, one of said resistance bridge networks beingconnected to said amplifier input circuit for impressing signals from atelephone line upon said input circuit to be amplified and the other ofsaid resistance bridge networks being connected to said amplifier outputcircuit for. impressing amplified signals from said output circuit upona telephone line, said resistance bridge networks each having a pair ofpoints having substantially zero potential therebetween, means forconnecting the corresponding zero potential points of said bridgenetworks together whereby signal transmission in the direction oppositeto that normally accomplished through said amplifier tube from the inputto the output thereof may be accomplished around said amplifier tube andsignal attenuating means in said last mentioned means for adjusting thelevel of signal transmission through said last mentioned means to thatdesired.

6. Noise suppression apparatus for telephone lines, comprising: apentode high gain amplifier tube having an input circuit and an outputcircuit, a pair of resistance bridge networks, one of said resistancebridge networks being connected to said amplifier input circuit forimpressing signals from a telephone line upon said input circuit to beamplified and the other of said resistance bridge networks beingconnected to said amplifier output circuit for impressing highlyamplified signals from said output circult upon a telephone line, saidresistance bridge networks'each having a pair of points havingsubstantially zero potential therebetween and signal attenuating meansfor connecting the corresponding zero potential points of said bridgenetworks together whereby signals transmission in the direction oppositeto that normally accomplished through said amplifier tube from the in-'put to the output thereof may be accomplished around said am lifiertube. 1

7. Noise 8111356581011 apparatus for telephone lines, comprising: anamplifier tube having an input circuit and an output circuit, a pair ofresistance bridge networks, a telephone line connected to one of saidbridge networks for impressing a telephone signal thereon, a transformerfor coupling said bridge network and said telephone line to said tubeinput circuit, for impressing signals from said telephone line upon saidinput circuit to be amplified, a second transformer for coupling theother of said resistance bridge networks to said amplifier outputcircuit for impressing amplified signals from said output circuit upon asecond telephone line, said resistance bridge networks each having apair of points having substantially zero potential therebetween andmeans for connecting the corresponding zero potential points of saidbridge networks together whereby signals transmission in the directionopposite to that normally vaccomplished through said amplifier tube fromthe input to the output thereof may be accomplished around saidamplifier tube.

8. Noise suppression apparatus for telephone lines, comprising: anamplifier tube having an input circuit and an output circuit, a pair ofresistance bridge networks, a telephone line connected to one of saidbridge networks for impressing a telephone signal thereon, a transformerfor coupling said bridge network'and said telephone line to said inputcircuit for impressing signals from said telephone line upon said inputcircuit to be amplified, a second transformer for coupling other of saidresistance bridge networks to said amplifier output circuit forimpressing amplified signals from said output circuit upon a secondtelephone line, said resistance bridge networks each having a pairof'points having substantially zero potential therebetween, means forconnecting the corresponding zero potentials points of said bridgenetworks together whereby signals transmission in the direction oppositeto that normally accomplished through said amplifier tube from the inputto the output thereof may be accomplished around said amplifier tube andsignal attenuating means in said last mentioned means for controllingthe level of signal transmission through sai ilast mentioned means. 7

9. Noise suppression apparatus for telephone lines, comprising: amicrophone, an amplifier tube having input and output circuits,connections between said microphone and said input circuit, a telephoneline connected to said output circuit for receiving amplified signalsfrom said amplifier output circuit, a telephone receiver, and aresistance pad connected between said telephone line and said telephonereceiver for attenuating the signals impressed upon said telephonereceiver from said telephone line and said amplifier output circuit.

10. Noise suppression apparatus for telephone lines, comprising: amicrophone, an amplifier tube having input and output circuits, atransformer for connecting said microphone to said input circuit, atransformer having a plurality of secondary windings for energizingcircuits of said microphone and said amplifier tube, a telephone lineconnected to said output circuit for receiving amplified signals fromsaid amplifier output circuit, a telephone receiver, a switch operableby said receiver, connections between said switch, a sourcenf currentsupply and the primary of said second transformer for connecting saidprimary to said source of current supply when said receiver is liftedfor use, and a resistance pad connected between said telephone line andsaid telephone receiver for attenuating the signals impressed upon saidtelephone receiver from said telephone line and said amplifier outputcircuit.

11. Noise suppression apparatus for telephone lines, comprising: amicrophone, an amplifier tube having input and output circuits,connections between said microphone and said input circuit, atransformer-having a plurality of secondary windings for energizingcircuits of said microphone and said amplifier tube, a telephone lineconnected to said output circuit for receiving amplified signals fromsaid amplifier output circuit, a telephone receiver, a switch operableby said receiver, connections between said switch, a

, be energized from said source of current supply,

said bell being connected to said telephone line and a switch forconnecting said source of ourrent supply to said telephone line forenergizing said bell and similar bells connected to said telephone line.

. 12. A telephone system adapted for high level signal transmission toreduce the eflects of noises produced in telephone lines, comprising: amicrophone, an amplifier tube having input and output circuits,connections between said microphone and said input circuit, a telephoneline connected to said output circuit for receiving highly amplifiedsignals from said amplifier output circuit, a telephone receiver, a.resistance pad connected between said telephone line and said telephonereceiver for attenuating the signals and noise impressed upon saidtelephone receiver to a lower level suflicient for the reproduction ofthe signals by said telephone receiver, a second amplifier having itsoutput circuit connected to said telephone line at a remote point forimpressing high level signals on said line atsaid remote point fortransmission to said telephone receiver, a resistance bridge networkconnected between said second amplifier and said telephone line, asecond telephone line adapted for lower level signal transmissionconnected to the input of said second amplifier and a circuit forimpressing the signals from said first telephone line on said secondtelephone line at an attenuated level.

13. Noise suppression apparatus for telephone lines, comprising: amicrophone, an amplifier having input and output circuits, saidmicrophone connected to said amplifier input circuit, a transformerhaving a primary connected to an alternating current power source, saidtransformer having a plurality of secondary windings,

a rectifier and filter connected between one of line, a resistance padconnected to said telephone line, a telephone receiver connected to saidresistance pad, a second amplifier having an output circuit connected tosaid telephone line at a point remote, from said first amplifier, and asecond telephone line connected to the input of said second amplifierfor feeding signals atrelatively low level to said second amplifierinput, said sec- 0nd amplifier being adjusted to feed high level signalsto said first telephone line to obtain a high signal to noise ratio sothat when the high level signals are attenuated at said resistance padto a level suitable for said telephone receiver the noise will besubstantially eliminated. I

BERGIE L. FISHER.

